1. Field of the Invention
The present invention relates to a process for producing dimethyl 2,6-naphthalene-dicarboxylate which is useful as a starting raw material for highly functional polyester.
2. Description of the Related Arts
As a process for producing dimethyl 2,6-naphthalene-dicarboxylate through the esterification of 2,6-naphthalene-dicarboxylic acid by the use of methanol, there have heretofore been known a process by using a mineral acid such as sulfuric acid as a catalyst (Japanese Patent Publication No. 174/1974), a process by using an oxide or a salt of any of various metals as a catalyst (Japanese Patent Application Laid-Open Nos. 83360/1975, 83361/1975, 8252/1976 and 48641/1976), a noncatalytic process in which the reaction is carried out at a temperature not lower than the critical temperature of methanol (Japanese Patent Application Laid-Open No. 95253/1975), a continuous esterification process (Japanese Patent Application Laid-Open Nos. 96574/1973 and Japanese Patent Publication No. 36179/1981), a process by using dimethyl phthalate as a solvent (Japanese Patent Application Laid-Open No. 200657/1990) and the like.
The above-mentioned processes are proposed as the methods of accelerating the reaction, since the rate of the esterification reaction is extremely low because of 2,6-naphthalene-dicarboxylic acid being sparingly soluble in methanol.
However, most of the above-mentioned processes are not necessarily satisfactory from the industrial viewpoint such as equipment and operation and is accompanied with various problems to be solved.
For example, the process by using a mineral acid such as sulfuric acid suffers the disadvantages that the equipment cost is undesirably increased by the necessity for corrosion resistance of the reactor and that a considerable amount of methanol is lost owing to the by-production of dimethyl ether accompanying the esterification reaction. In addition, the process by using a metallic compound as a catalyst, the noncatalytic process in which the reaction is carried out at a temperature not lower than the critical temperature of methanol and the like process are still unsatisfactory from the industrial viewpoint such as the rate of reaction.
Moreover, the continuous esterification process involves the problems that the sparingly methanol-soluble 2,6-naphthalene-dicarboxylic acid necessitates handling it as a solid, and clogging or the like is caused in the production equipment or piping because of the high melting point of dimethyl 2,6-naphthalene-dicarboxylate that is the esterified product, whereby special equipment and methods are made necessary in order to operate the production equipment in a stable manner.
Further, the process by using dimethyl phthalate as a solvent involves such problems as the corrosion resistance of the reactor and the by-production of dimethyl ether as mentioned above due to the use of sulfuric acid as a catalyst.